A discharge lamp, such as a cold cathode fluorescent lamp (CCFL), has terminal voltage characteristics that vary depending upon the immediate history and the frequency of a stimulus (AC signal) applied to the lamp. Until the CCFL is “struck” or ignited, the lamp will not conduct a current with an applied terminal voltage that is less than the strike voltage. Once an electrical arc is struck inside the CCFL, the terminal voltage may fall to a run voltage that is approximately ⅓ of the strike voltage over a relatively wide range of input currents. When the CCFL is driven by an AC signal at a relatively high frequency, the CCFL (once struck) will not extinguish on each cycle and will exhibit a positive resistance terminal characteristic. Since the CCFL efficiency improves at relatively higher frequencies, the CCFL is usually driven by AC signals having frequencies that range from 50 KiloHertz to 100 KiloHertz.
Driving a CCFL with a relatively high frequency square-shaped AC signal will produce the maximum useful lifetime for the lamp. However, since the square shape of an AC signal may cause significant interference with other circuits in the vicinity of the circuitry driving the CCFL, the lamp is typically driven with an AC signal that has a less than optimal shape such as a sine-shaped AC signal.
Most small CCFLs are used in battery powered systems, e.g., notebook computers and personal digital assistants. The system battery supplies a direct current (DC) voltage ranging from 7 to 20 Volts with a nominal value of about 12V to an input of a DC to AC inverter. A common technique for converting a relatively low DC input voltage to a higher AC output voltage is to chop up the DC input signal with power switches, filter out the harmonic signals produced by the chopping, and output a relatively clean sine-shaped AC signal. The voltage of the AC signal is stepped up with a transformer to a relatively high voltage, e.g., from 12 to 1500 Volts. The power switches may be bipolar junction transistors (BJT) or field effect transistors (MOSFET). Also, the transistors may be discrete or integrated into the same package as the control circuitry for the DC to AC converter.
Since resistive components tend to dissipate power and reduce the overall efficiency of a circuit, a typical harmonic filter for a DC to AC converter employs inductive and capacitive components that are selected to minimize power loss, i.e., each of the selected components should have a high Q value. A second-order resonant filter formed with inductive and capacitive components is also referred to as a “tank” circuit because the tank stores energy at a particular frequency.
The electronic device incorporating the CCFL often is used in a wide variety of environmental conditions, such as wide temperature variations. Further, variations in the values of the components both within the driving circuit and in external components normally occur. Because of this, the amount of energy needed to most efficiently strike the CCFL may vary.